This application is based on Japanese Patent Application No. 2000-237724 filed in Japan on Jun. 30, 2000, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display element.
2. Description of the Related Art
In recent years, research has been carried out regarding various types of liquid crystal display elements that use liquid crystal exhibiting a cholesteric phase. Chiral nematic liquid crystal comprising nematic liquid crystal to which a chiral dopant is added such that the liquid crystal exhibits a cholesteric phase at room temperature is one of the representative cholesteric liquid crystal materials.
Such a liquid crystal display element generally has a construction in which a liquid crystal layer including liquid crystal that exhibits a cholesteric phase is held between a pair of substrates, and is used, for example, as a reflective liquid crystal display element that takes advantage of the selective reflection property of the liquid crystal that exhibits a cholesteric phase. Display is carried out in a reflective liquid crystal display element of this type by alternating the liquid crystal from a planar state (colored state) to a focal conic state (non-colored state) and vice versa through the application of a high or low voltage.
Incidentally, an orientation film may be used in such a liquid crystal display element in order to control the orientation of the liquid crystal molecules.
However, when an orientation film is used in a conventional reflective liquid crystal display element using liquid crystal that exhibits a cholesteric phase, if the interaction between the liquid crystal and the orientation film is not sufficiently strong, the liquid crystal molecules are not uniformly controlled by the orientation effect, so that diffusion based on the focal conic state increases and sufficient contrast is not obtained. It is possible to perform rubbing the orientation film (i.e., a process in which the surface of the orientation film is rubbed in one direction using nylon cloth, for example) in order to increase the interaction between the liquid crystal and the orientation film, but if that is done, the element""s dependence on the angle of view increases.
An object of the present invention is therefore to provide an improved liquid crystal display element comprising a liquid crystal layer that is held between a pair of substrates and includes liquid crystal that exhibits a cholesteric phase.
Another object of the present invention is to provide a liquid crystal display element comprising a liquid crystal layer that is held between a pair of substrates and includes liquid crystal that exhibits a cholesteric phase, wherein the orientation effect can uniformly control the molecules of the liquid crystal, enabling the display of high-contrast images.
Still another object of the present invention is to provide a liquid crystal display element comprising a liquid crystal layer that is held between a pair of substrates and includes liquid crystal that exhibits a cholesteric phase, wherein the element""s dependence on the angle of view may be reduced.
For a liquid crystal display element comprising (1) a liquid crystal layer held between a pair of substrates and including liquid crystal exhibiting a cholesteric phase and (2) an orientation film on at least one of the substrates, it has been found that the introduction of a mesogenic group (i.e., a substitutional group that has a molecular interaction with the molecules of the liquid crystal) into the orientation film causes increased interaction between the liquid crystal and the orientation film, thereby resulting in a uniform orientation effect extending to the liquid crystal molecules, reduced diffusion when the focal conic state is present, and improved contrast in the display images. It has also been found that the element""s dependence on the angle of view may be reduced without the need for rubbing of the orientation film.
The present invention was created based on this knowledge, and in order to resolve the above problems, a liquid crystal display element reflecting one aspect of the present invention comprises a liquid crystal layer that is held between a pair of substrates and includes liquid crystal that exhibits a cholesteric phase, wherein at least one of the substrates has an orientation film that comprises a polyimide having a mesogenic group in its structure and the orientation film has not undergone rubbing.
The mesogenic group referred to in the present specification means a substitutional group having a rigid structure necessary for the molecules to exhibit a liquid crystal nature. Such mesogenic groups include, for example, a cholesterol group, a biphenyl group or a phenylbenzoate group.
The liquid crystal display element pertaining to the above mentioned structure may be used as a reflective liquid crystal display element that takes advantage of the selective reflection property of the liquid crystal that exhibits a cholesteric state.
When the liquid crystal display element pertaining to the above mentioned structure is used as a reflective liquid crystal display element, display is performed by alternating the liquid crystal between a planar state (colored state) and a focal conic state (non-colored state) through the application of a high or low voltage.
Because the orientation film comprises a polyimide having a mesogenic group in its structure, when the liquid crystal display element pertaining to the above mentioned structure is used, the interaction between the liquid crystal and the orientation film increases, so that the orientation effect may be uniformly extended to the molecules of the liquid crystal and the contrast of the image display may be improved accordingly. In addition, because the orientation film does not undergo rubbing, the element""s dependence on the angle of view may be reduced.
The mesogenic group referred to above may belong to any of the following groups:
(a) A mesogenic group that includes a steroid skeleton, biphenyl skeleton or phenylcyclohexane skeleton;
(b) A side chain of a diamine compound that forms a polyimide; and
(c) A combination of (a) and (b) above.
The chemical formulae of the compounds from which the material of the orientation film may be synthesized are shown in (1) through (5) below. 
Any of the following films may be used as the orientation film, for example:
(a) A film comprising a polyimide that is synthesized from the compounds indicated by the chemical formulae (1), (2) and (3) and that has in its structure a mesogenic group including a steroid skeleton;
(b) A film comprising a polyimide that is synthesized from the compounds indicated by the chemical formulae (1), (2) and (4) and that has in its structure a mesogenic group including a biphenyl skeleton; or
(c) A film comprising a polyimide that is synthesized from the compounds indicated by the chemical formulae (1), (2) and (5) and that has in its structure a mesogenic group including a phenylcyclohexane skeleton.
In any event, the liquid crystal that exhibits a cholesteric phase may be chiral nematic liquid crystal comprising nematic liquid crystal to which a chiral dopant is added. In this case, it is preferred that the chiral dopant be added in the range between 7 percent by weight and 50 percent by weight, for example.
Where chiral nematic liquid crystal comprising nematic liquid crystal to which a chiral dopant is added is used as the liquid crystal that exhibits a cholesteric phase, the benefit obtained is that the selective reflection wavelength of the chiral nematic liquid crystal may be controlled by changing the amount of the chiral dopant added. If the amount of the chiral dopant added is too small, a sufficient memory capability (i.e., maintenance of the planar state in the area that was in the planar state or maintenance of the focal conic state in the area that was in the focal conic state when the application of voltage was stopped) may not be obtained. If the amount of the chiral dopant is too large, the liquid crystal may no longer exhibit a cholesteric phase at room temperature or may solidify.
In any event, if the orientation film is too thin, the orientation of the liquid crystal molecules may not be controlled, and if it is too thick, the drive voltage increases. The thickness of the orientation film is preferably between 100 xc3x85 and 2,000 xc3x85, for example.
Two or more liquid crystal display elements pertaining to the present invention may be stacked together to form a multi-layered liquid crystal display element. In this case, color display using two or more colors may be performed by stacking together two or more liquid crystal display elements that perform display of different colors. If at least three liquid crystal display elements, which perform blue display, green display and red display, respectively, are used in a multi-layered liquid crystal element, full-color display may be performed.
In any event, in such a multi-layered liquid crystal display element, one substrate may be shared between two adjacent liquid crystal display elements.